At present, door closers are generally classified into mechanical door closers, pneumatic door closers and hydraulic door closers, wherein mechanical door closers generally employ an elastic assembly formed by a spring. When an operator pushes a door or window, a drive spring accumulates energy through deformations such as stretching, compression or torsion, and the accumulated energy will be released when the operator moves away from the door or window. A pull rod, driven by the spring, closes the door. However, ordinary mechanical door closers in the prior art cannot be positioned in use and cannot enable the door to remain at a certain opening angle. When articles are transported through the door or many people go through the door in a short period of time, collision between the door and articles or people might occur, thereby causing inconvenience. To solve this problem, some people achieves the positioning by improving the internal structure of the door closer, but the improvement is complicated in structure and assembling. As far as a user is concerned, the improvement requires the user to replace the whole of the original door closer, which increases the costs.